1. Field of the Invention
The present invention relates to an active matrix type of twisted nematic liquid crystal display in which large pieces of lights are circularly polarized by pieces of liquid crystal arranged in an orderly matrix of picture elements to display a colored image.
2. Description of the Related Art
A liquid crystal display is conventionally utilized to display a colored image. In the liquid crystal display, liquid crystal is arranged along an optical path of a piece of incident light while twisting an orientation of the liquid crystal. Therefore, the incident light is circularly polarized in the liquid crystal display.
Also, an active matrix type of twisted nematic liquid crystal display in which a large number of picture elements are arranged in an orderly matrix has been recently utilized as one of the liquid crystal displays. In each of the picture elements of the active matrix type of twisted nematic liquid crystal display, liquid crystal set in a nematic phase is filled up to twist the orientation of the liquid crystal along the optical path, and a switching device is equipped to control the orientation of the liquid crystal. The twisted nematic liquid crystal display can be easily manufactured in a thin shape, and electric power consumed to operate the twisted nematic liquid crystal display is low. In addition, a contrast between a bright light and a dark light is superior in the twisted nematic liquid crystal display. Therefore, a colored image can be displayed in a high contrast on the twisted nematic liquid crystal display. Accordingly, the twisted nematic liquid crystal display has been developed for use as a television display in place of a cathode-ray tube of the television.
2.1. Previously Proposed Art
The active matrix type of twisted nematic liquid crystal display conventionally utilized is briefly described with reference to the drawings.
FIG. 1 is a plan view of the matrix type of twisted nematic liquid crystal display conventionally utilized.
As shown in FIG. 1, the matrix type of twisted nematic liquid crystal display 10 is provided with an array substrate 11 formed of transparent glass material, a large number of gate lines 12 arranged in parallel with each other on the array substrate 11, and a large number of source lines 13 arranged at right angles to the gate lines 12 on the array substrate 11. Therefore, a large number of picture elements 14 which each are surrounded by both one of the gate lines 12 and one of the source lines 13 are formed on the array substrate 11 in an orderly matrix.
In the above configuration, a pulse signal is transmitted through each of the gate lines 12, and a changeable electric potential is applied to the source lines 13.
FIG. 2 is a plan view of one of the picture elements 14 of the twisted nematic liquid crystal display 10 shown in FIG. 1. FIG. 3 is a sectional view of the picture element 14 shown in FIG. 2, the sectional view being taken generally along the line III--III of FIG. 2.
As shown in FIGS. 2, 3, each of the picture elements 14 is provided with the array substrate 11, a plate type of lower polarizer 15 arranged just under the array substrate 11 for linearly polarizing an incident white light with which the picture element 14 is irradiated, a thin type of amorphous silicon transistor 16 turned on by the pulse signal transmitted from the gate line 12, a picture element electrode 17 (or a pixel electrode 17) arranged on the array substrate 11 for receiving the changeable electric potential transmitted from the source line 13 through the amorphous silicon transistor 16 turned on by the pulse signal, and a passivation film 18 arranged between the source line 13 and the picture element electrode 17 for insulating the picture element electrode 17 from the source line 13.
The picture element electrode 17 is formed of an indiumtin oxide thin film so that the electrode 17 is transparent.
Also, the passivation film 18 is transparent.
In the above configuration, when a pulse signal is transmitted to the gate line 12, the amorphous silicon transistor 16 is turned on by the pulse signal as shown in FIG. 4 so that the changeable electric potential applied in the source line 13 is applied to the picture element electrode 17 through the amorphous silicon transistor 16 turned on. Thereafter, even though the amorphous silicon transistor 16 is turned off, the picture element electrode 17 is maintained to the changeable electric potential because the picture element electrode 17 is insulated by the passivation film 18. Also, the lower polarizer 15 is irradiated with the incident white light functioning as a back light. In this case, the incident white light is linearly polarized in a first polarizing direction PD1.
Also, each of the picture elements 14 is further provided with a lower alignment layer 19 arranged on the passivation film 18 in which polymers are oriented in a first rubbing direction RD1, an upper alignment layer 20 arranged over the lower alignment layer 19 through a spacer formed of glass beads in which polymers are oriented in a second rubbing direction RD2 normal to the first rubbing direction RD1, and a liquid crystal layer 21 arranged between the lower and upper alignment layer 19, 20.
A distance between the lower and upper alignment layers 19, 20 ranges from 2 .mu.m to 10 .mu.m.
The lower and upper alignment layers 19, 20 are transparent.
In the above configuration, liquid crystal filled in the liquid crystal layer 21 is crystallized in a nematic type of orientation. Also, the liquid crystal is a dielectric substance having an anisotropic dielectric constant, and the anisotropic dielectric constant is positive. In addition, a surface of the lower alignment layer 19 facing to the upper alignment layer 20 is rubbed to orient the polymers of the film 19 in the first rubbing direction RD1, and a surface of the upper alignment layer 20 facing to the lower alignment layer 19 is rubbed to orient the polymers of the film 20 in the second rubbing direction RD2. That is, when the alignment layers 19, 20 are rubbed in the rubbing directions RD1, RD2, a large number of small grooves are produced so that the polymers are extended towards the rubbing directions.
The rubbing directions RD1, RD2 of the alignment layers 19, 20 are set at 45 degrees to the gate and source lines 12, 13 as shown in FIG. 2 according to a conventional rubbing process. The reason that the rubbing directions RD1, RD2 are set at 45 degrees to the gate and source lines 12, 13 is to uniformly adjust the contrast of the image displayed on the entire picture element 14.
Therefore, when the liquid crystal is filled in a region between the lower and upper alignment layers 19, 20 after the lower and upper alignment layers 19, 20 are attached to each other through the spacer, major axes of liquid crystal molecules in the liquid crystal directly attached on the alignment layer 19 are oriented in the rubbing direction RD1 of the film 19 because the liquid crystal molecules oriented in the rubbing direction RD1 is energetically stable on the alignment layer 19. Also, major axes of liquid crystal molecules directly attached on the alignment layer 20 are oriented in the rubbing direction RD2 of the film 20 because the liquid crystal molecules oriented in the rubbing direction RD2 is energetically stable on the alignment layer 20.
Accordingly, the major axes of the liquid crystal molecules are gradually twisted like a spiral staircase along the optical path of the incident white light oriented from the lower alignment layer 19 to the upper alignment layer 20. As a result, the liquid crystal molecules are twisted 90 degrees in the liquid crystal layer 21. Therefore, in cases where any electric potential is not applied to the picture element electrode 17, the incident white light is circularly polarized 90 degrees in the liquid crystal layer 21 after the liquid crystal layer 21 is irradiated with the incident white light linearly polarized by the lower polarizer 15.
The major axes of the liquid crystal molecules are uniformly twisted either in a clockwise direction or in a counterclockwise direction in the entire liquid crystal layer 21 of the twisted nematic liquid crystal display 10.
In addition, the major axes of liquid crystal molecules are tilted in an upper direction at a pretilt angle .theta. having a tilt value. As shown in FIG. 5, the pretilt angle .theta. of a liquid crystal molecule is generally defined as an angle between the surface of the alignment layer 19 or 20 and a major axis of the liquid crystal molecule. The arrangement of the liquid crystal molecules at the pretilt angle .theta. is generally generated by a repulsive dipole-dipole interaction between a dipole of the polymer and another dipole of the liquid crystal molecule. Also, there are two types of twist conditions. One condition is called a normal twist condition, and another condition is called a reverse twist condition. The tilt value of the pretilt angle generally ranges from about 1 degree to 5 degrees in the normal twist condition.
Also, each of the picture elements 14 is further provided with a common electrode 22 for generating an electric field between the picture element electrode 17 charged to the changeable electric potential and the common electrode 22, a colored filter 23 arranged on the common electrode 22 for coloring the incident white light passing through the liquid crystal layer 21 red, green or blue (RGB), an opposite substrate 24 formed of transparent glass material for mounting the common electrode 22 and the colored filter 23, and a plate type of upper polarizer 25 for linearly polarizing colored light produced in the colored filter 23 in a second polarizing direction PD2 normal to the first polarizing direction PD1 of the lower polarizer 15.
The common electrode 22 is formed of an indium-tin oxide thin film so that the electrode 22 is transparent.
In the above configuration, in cases where any electric potential is not applied to the picture element electrode 17, no electric field is generated between the element picture electrode 17 and the common electrode 22. Therefore, the major axes of the liquid crystal molecules remain twisted 90 degrees in the liquid crystal layer 21. In this case, the incident white light is circularly polarized 90 degrees in the liquid crystal layer 21 after the incident white light is linearly polarized in the first polarizing direction PD1 by the lower polarizer 15. Therefore, the incident white light passing through the liquid crystal layer 21 is linearly polarized in the same direction as the second polarizing direction PD2 of the output polarizer 25 normal to the first polarizing direction PD1. Thereafter, the incident white light is colored red, green or blue in the colored filter 23 to produce a piece of colored light, and the colored light produced is linearly polarized in the second polarizing direction PD2. In this case, because the colored light has been linearly polarized in the same direction as the second polarizing direction PD2 in the liquid crystal layer 21, quantity of the colored light is not varied in the upper polarizer 25.
Accordingly, the colored light is radiated from each of the picture elements 14.
In cases where the picture element 14 having the red-colored filter 23, the picture element 14 having the green-colored filter 23, and the picture element 14 having the blue-colored filter 23 are arranged in a set, a viewer can feel colored lights provided from the above three picture elements 14 as a white light.
In contrast, in cases where a changeable electric potential is applied to the picture element electrode 17 from the source line 13, a vertical electric field is generated between the element picture electrode 17 and the commonelectrode 22. In this case, the major axes of the liquid crystal molecules twisted 90 degrees in the liquid crystal layer 21 are forcibly oriented in the same direction as the vertical electric field on condition that the intensity of the vertical electric field is high enough to forcibly orient the major axes of the liquid crystal molecules in a vertical direction parallel to the vertical electric field. Therefore, the incident white light is not circularly polarized at all in the liquid crystal layer 21 after the incident white light is linearly polarized in the first polarizing direction PD1 by the lower polarizer 15. Thereafter, the incident white light not circularly polarized is colored red, green or blue in the colored filter 23 to produce a colored light, and the colored light is transmitted to the second polarizing direction PD2. In this case, because the colored light is linearly polarized in the first polarizing direction PD1, the incident white light can not pass through the upper polarizer 25 because the colored light is linearly polarized in the second polarizing direction PD2 normal to the first polarizing direction PD1 by the upper polarizer 25. Therefore, the incident white light radiated to the picture element 14 can not transmit through the picture element 14.
Accordingly, in cases where the picture element 14 having the red-colored filter 23, the picture element 14 having the green-colored filter 23, and the picture element 14 having the blue-colored filter 23 are arranged in a set, one of three light source colored lights can be obtained by applying the changeable electric potential to two of the three picture elements 14 arranged in a set.
Also, in cases where the changeable electric potential applied to the source line 13 is controlled, major axis directions of the liquid crystal molecules of which major axes are twisted 90 degrees can be changed by a suitable angle. Therefore, the incident white light can be circularly polarized in the liquid crystal layer 21, dependent on the intensity of the changeable electric potential. As a result, the quantity of the colored light transmitted from the picture element can be controlled.
Accordingly, any type of colored light can be obtained by combining a piece of red light, a piece of green light, and a piece of blue light radiated from the set of three picture elements 14. Therefore, a colored image can be displayed on the matrix type of twisted nematic liquid crystal display 10.
2.2 Problems to be Solved by the Invention
However, the gate line 12 and the source line 13 is arranged adjacent to the picture element electrode 17 because a large number of picture elements 14 must be arranged in close formation to display a fine image. For example, the distance between the gate or source line 12, 13 and the picture element electrode 17 is within several micro meters (.mu.m).
Also, the electric potential applied to the source line 13 is always changed to control the polarization direction of the incident white light circularly polarized in the liquid crystal layer 21 by changing major axis directions of the liquid crystal molecules. Therefore, when the transistor 16 is turned off, an electric potential difference between the picture element electrode 17 and the source line 13 is necessarily generated, dependent on the change of the electric potential applied to the source line 13. As a result, not only the vertical electric field is induced between the picture element electrode 17 and the common electrode 22, but also a first lateral electric field is induced between the picture element electrode 17 and the source line 13.
In addition, the electric potential of the gate line 12 is always changed because a pulse signal is intermittently transmitted in the gate line 12. Therefore, another electric potential difference between the picture element electrode 17 and the gate line 12 is necessarily generated. As a result, a second lateral electric field is induced between the picture element electrode 17 and the gate line 12. The first and second lateral electric fields are called a lateral electric field for convenience in this specification hereinafter.
Because the lateral electric field is inevitably generated in each of the picture elements 14 in the twisted nematic liquid crystal display 10, even though the alignment layers 19, 20 are rubbed in the rubbing directions RD1, RD2 normal to each other, the liquid crystal molecules in the liquid crystal layer 21 are irregularly oriented. In other words, the normal tilt condition of the liquid crystal molecules is changed to the reverse tilt condition to stably arrange the liquid crystal molecules. Therefore, even though no vertical electric field is induced between the picture element electrode 17 and the common electrode 22, the major axes of the liquid crystal molecules in the liquid crystal 21 are not accurately twisted 90 degrees so that the contrast of the image displayed on the twisted nematic liquid crystal display 10 deteriorates.
Also, because the rubbing direction RD1 of the alignment layer 19 is generally set at 45 degrees to the gate and source lines 12, 13 and because the major axes of the liquid crystal molecules are twisted from the rubbing direction RD1 to the rubbing direction RD2, adverse influence of the lateral electric field such as a dipole-dipole interaction is easily affected on the liquid crystal molecules.
Also, because the tilt value of the pretilt angle is small, a dipole moment adversely influenced on each of the liquid crystal molecules by the lateral electric field is comparatively large.
Also, because the major axes of the liquid crystal molecules are uniformly twisted either in a clockwise direction or in a counterclockwise direction, the elastic deformation of the liquid crystal molecules is not enough to change the liquid crystal molecules irregularly oriented in the liquid crystal layer 21 to the liquid crystal molecules regularly oriented.
As a result, the liquid crystal molecules are irregularly oriented in the liquid crystal layer 21 adjacent to the picture element electrode 17 by the lateral electric field.
In addition, as a result of the liquid crystal molecules irregularly oriented in the liquid crystal layer 21, a reverse tilt disclination line is generated in the liquid crystal layer 21 adjacent to the picture element electrode 17. In cases where the reverse tilt disclination line is generated, even though any electric potential is not applied to the picture element electrode 17 by the source line 17, the incident white light polarized by the lower polarizer 15 can not be circularly polarized by liquid crystal molecules arranged adjacent to the reverse tilt disclination line so that any colored light can not be radiated from the picture element 14. Also, even though an electric potential is applied to the picture element electrode 17 by the source line 17, the incident white light polarized by the lower polarizer 15 is circularly polarized to some extent by liquid crystal molecules arranged adjacent to the reverse tilt disclination line so that a piece of colored light is radiated from the picture element 14. Therefore, the contrast of the image deteriorates.
The mechanism that the reverse tilt disclination line is generated by the lateral electric field induced between the picture element electrode 17 and the gate line 12 or between the picture element electrode 17 and the source line 13 is described with reference to FIG. 6.
FIGS. 6A, 6B explanatorily show the diffusion of the liquid crystal molecules set to the reverse tilt condition.
In cases where the intensity of the lateral electric field is low, liquid crystal molecules arranged adjacent to the picture element electrode 17 are regularly oriented. In other words, the liquid crystal molecules directly arranged on the lower alignment film 19 are set in the normal tilt condition as shown in FIG. 5. Therefore, the liquid crystal molecules in the liquid crystal layer 21 are set in a normal twist condition.
In contrast, in cases where the intensity of the lateral electric field is high, liquid crystal molecules arranged adjacent to edges of the picture element electrode 17 are irregularly oriented because the lateral electric field is concentrated at the edges of the picture element electrode 17. In other words, the liquid crystal molecules are set in the reverse tilt condition as shown in FIG. 5. Therefore, other liquid crystal molecules arranged on the liquid crystal molecules which are arranged adjacent to the edges of the picture element electrode 17 are set in a reverse twist condition.
Therefore, as shown in FIG. 6A, when the intensity of the lateral electric field becomes high, liquid crystal molecules arranged adjacent to edges of the picture element electrode 17 are strongly set in the reverse tilt condition. In contrast, liquid crystal molecules arranged on the picture element electrode 17remain set in the normal tilt condition.
Thereafter, as shown in FIG. 6B, the liquid crystal molecules strongly set in the reverse tilt condition is changed to the liquid crystal molecules weakly set in the reverse tilt condition while the liquid crystal molecules set in the normal tilt condition on the picture element electrode 17 is changed to the liquid crystal molecules weakly set in the reverse tilt condition. Therefore, the reverse tilt condition is diffused to the liquid crystal molecules arranged on the picture element electrode 17.
Thereafter, when a vertical electric field is induced between the picture element electrode 17 and the common electrode 24, major axes of the liquid crystal molecules weakly set in the reverse tilt condition are vertically oriented along the vertical electric field on the picture element electrode 17 while the liquid crystal molecules remain set in the reverse tilt condition. In contrast, major axes of other liquid crystal molecules set in the normal tilt condition are vertically oriented along the vertical electric field on the picture element electrode 17 while the liquid crystal molecules remain set in the normal tilt condition.
As a result, the reverse tilt disclination line is generated in a boundary line between the liquid crystal molecules set to the reverse tilt condition and the liquid crystal molecules set to the normal tilt condition.
Accordingly, the incident white light passing through the liquid crystal molecules arranged adjacent to the reverse tilt disclination line is irregularly poralized regardless of whether an electric potential is applied to the picture element electrode 17. Therefore, the contrast of the image displayed on the twisted nematic liquid crystal display 10 deteriorates.
In addition, a shielding film is generally equipped in a display domain of the picture element 14 to shield the incident white light passing through the liquid crystal molecules arranged adjacent to the reverse tilt disclination line. In this case, the incident white light passing through the liquid crystal molecules arranged adjacent to the reverse tilt disclination line can not transmit through the picture element 14 regardless of whether or not an electric potential is applied to the picture element electrode 17. Therefore, the contrast is improved. However, the ratio of an opening area which is not shielded by the shielding film to the entire area of the picture element 14 deteriorates so that the image can not displayed in a high brightness.